In mobile networks, such as 3GPP networks (3GPP: Third Generation Partnership Project) there is a continuing need for higher throughputs. In uplink (UL) communication, i.e., in communication from a mobile terminal to the network, transmit (Tx) power limitations in mobile terminals and constraints on the UL budget mean that such higher throughputs typically necessitate smaller cell sizes than is typically deployed for present cellular systems. For example, in heterogeneous networks, also referred to as HetNet, conventional base stations (BSs), referred to as Macro BSs, are complemented by further BSs, which are usually deployed closer to the end users, e.g., on street level. As compared to the Macro BSs, these further BSs typically transmit at a lower power. In the following, these further BSs will also be referred to as Pico BSs. The Pico BSs can use carrier frequencies from the same spectrum as the Macro BSs or may use carrier frequencies from another spectrum.
In current cellular networks, e.g., 3GPP Long Term Evolution (LTE), selection of a serving BS, also referred to as cell association, is based on a Reference Signal Received Power (RSRP) measurement by the mobile terminal. That is to say, the respective RSRPs as measured for different BSs are used as a basis for selecting a BS to act as the serving BS of the mobile terminal. If the selected serving BS is different from the present serving BS of the mobile terminal, a handover of the mobile terminal to the selected BS may be initiated. The serving BS typically sends downlink (DL) transmissions to the mobile terminal and receives UL transmissions from the mobile terminal.
The RSRP depends on the Tx power of the respective BS. In the case of a heterogeneous network deployment, there may be a large imbalance in the Tx power between a Macro BS and a Pico BS. Specifically, Macro BSs usually transmit at a high Tx power level, e.g., 46 dBm, whereas Pico BSs may use a much lower Tx power level, e.g., 30 dBm, or even less. Thus, the difference in Tx power between transmitted reference signals can be 16 dB or more. The RSRP is indicative of a DL channel quality between the BS and the mobile terminal. As compared to that, the UL channel quality between the mobile terminal and the BS depends on the signal damping, also referred to as pathloss, between the mobile terminal and the BS.
In conventional homogeneous networks, the BS that provides the highest RSRP is typically identical to the BS that yields the lowest average pathloss. However, this is not necessarily the case in heterogeneous networks. The imbalance in the Tx power among the different types of BSs may lead to a situation where a mobile measures a higher RSRP from a Macro BS, although it is located closer to a Pico BS and its pathloss to the Pico BS is smaller than the pathloss to the Macro BS. In this case, it would be preferable for the mobile terminal to receive data from the BS with the highest RSRP, i.e., from the Macro BS. On the other hand, it would be preferable for the mobile terminal to transmit data to the BS with the lowest pathloss, i.e., to the Pico BS. Accordingly, in a heterogeneous network conventional RSRP-based BS selection may result in suboptimal performance. Imbalances between DL and UL performance may however also exist in homogeneous networks.
Accordingly, in heterogeneous networks as mentioned above, but also in homogeneous networks, there is a need for techniques which allow for efficiently balancing performance of a mobile terminal in the UL and the DL.